1. Field of the Invention
The present invention relates to the treatment of subterranean formations penetrated by wellbores, and relates more particularly to methods for treatment of subterranean siliceous formations with acid.
2. Prior Art
Often the cause of low productivity of oil and gas in subterranean sandstone formations is reduced formation permeability near the wellbore. This condition, called "formation damage" has been related to a variety of completion and drilling practices. For example, the perforating of casing may reduce permeability around the perforation by matrix crushing and compaction caused by the shaped charge or by gun debris. The loss of completion fluids, filtrates from drilling mud, or drilling mud particles may cause clay swelling, particle plugging by dispersed formation fines, particle invasion, or adverse change in fluid saturation.
Many of the adverse permeability effects relate directly to the clay fraction of the matrix. It is well known that certain effects such as clay swelling, clay dispersion, and particle plugging can be reduced by solubilizing the clay with mineral acid solutions of hydrogen fluoride. Commonly, aqueous solutions containing from about 2 to 6 weight percent hydrofluoric acid and from 5 to 15 weight percent hydrochloric acid (generally called "Mud Acid") are employed to treat the damage formations. The low pH conditions provided by the hydrochloric acid is beneficial in solubilizing the products formed by the reaction between hydrofluoric and the formation material. This HF-HCl treating solution reacts most rapidly with calcite, rapidly with clay, less rapidly with other silicates, and slowest with the silica of natural sands. The precise rates of these reactions are largely controlled by the concentration of hydrofluoric acid in the treating solution. The higher the hydrofluoric acid concentration the more rapid the reaction rate.
Because the reaction of hydrofluoric acid on silica and clay is generally rapid at formation temperatures (120-220.degree. F.), a majority of the acid solution becomes spent within a radius of about 24 inches from the wellbore and a lesser amount of active acid solution penetrates beyond this distance. Moreover, because the majority of the reaction occurs within a small volume of the formation matrix, the concentration of reaction products (some of which may precipitate) and fines liberated by the reaction may cause permeability impairment if the hydrofluoric acid concentration is too high.
Because the materials plugging the porous formation of rock are only removed for a short distance around the well, invasion of the treated region by new fine particles can occur after a short period of time. These fine particles are believed to originate deeper in the formation and are carried toward the wellbore by the flowing fluid during production. A method for achieving deeper acid penetration would remove these particles to a greater radius from the wellbore, thereby lengthening the time required for fines to migrate back to the well.
Localized spending of hydrofluoric acid near the wellbore may also result in increased water production. Hydrofluoric acid injected into the wellbore reacts readily at the interface between the formation rock and the cement used to set the wellbore casing. A channel may be formed along this interface which can allow water from the nearby strata to reach the producing well. Elimination of this problem requires that the concentration of hydrofluoric acid in solution be low as fluids are injected into the formation past the formation-cement interface.
A plastic consolidation of incompetent sands is strongly effected by the quality of any preceding acid treatment. The compressive strength of a plastic consolidation decreases as the clay content of the sand increase. It is observed that the strength profile of the consolidation parallels the clay removal profile by the acid. To have high consolidation strengths over the formation volume to be consolidated, clay should be thoroughly and evenly removed from that volume.
Numerous procedures have been suggested to obtain substantial penetration of hydrofluoric acid into the formation. One method comprises contacting an oil-containing sandstone formation with a surface-active compound to render the surfaces of the formation sands oil-wet, thereby imparting hydrophobic properties to the formation. As a result of the formation having hydrophobic properties, the reaction rate of hydrofluoric acid within the formation is retarded, thereby enhancing acid penetration into the formation.
Another method, as described in U.S. Pat. No. 3,828,854, issued to Templeton et al on Aug. 13, 1974, generates hydrofluoric acid in situ. In this method, an aqueous solution of a water soluble fluoride salt is mixed with a relatively slowly-reactive acid-yielding material that subsequently converts the fluoride salt solution to a hydrofluoric acid solution that has a relatively high pH (at least about 2), but is capable of dissolving siliceous materials. In the preferred embodiment, the fluoride salt is an ammonium salt of hydrofluoric acid and the acid-yielding material is a formic acid ester. One problem with this method is that the formic acid formed from the ester hydrolysis is a weak acid so that the pH of the treating solution formed is relatively high. In this relatively high pH solution, the hydrofluoric acid concentration is relatively low so that the rate and the extent of the reaction with the formation matrix is consequently low. For any fluoride ion concentration, the pH should be reduced to at least about 1 to maximize hydrofluoric acid generation from fluoride ion and to effectively solubilize reaction products. This process is further limited by the solubility of the formic acid ester in ammonium fluoride solutions. The total amount of hydrofluoric acid which can be formed by this process is limited by the amounts of formic acid and ammonium fluoride which can be combined in solution.
Another hydrofluoric acid generation method (as described in SPE Paper No. 6512 entitled "A New Technique for Generating In-Situ Hydrofluoric Acid for Deep Clay Damage Removal" by B. E. Hall, presented at the 47th Annual California Regional Meeting of the SPE on Apr. 13-15, 1977) is multi sequential injection of fluoride ion solution followed by a hydrochloric acid solution. The beneficial effect of this procedure is ascribed to the adsorption of fluoride ions on the anion exchange sites of the clay minerals followed by activation of the adsorbed fluoride ions by a pursuing hydrochloric acid solution. The effectiveness of this process is limited by the anion exchange capacity of the resident clay. Since most clays have a small anion exchange capacity, the amount of hydrogen fluoride produced by this method is also small.
A need still exists for an improved technique for generating hydrofluoric acid in a formation which overcomes the problems associated with rapid spending of the acid solution within a short radial distance from the wellbore.